Studies proposed in this application are a continuation of ongoing investigations into the molecular basis for immune recognition or failure of recognition of tumor-associated antigens. The model system chosen for study employs mouse cells transformed by SV40 virus. Previous studies have implicated SV40-specific cytotoxic T lymphocytes (CTL) in the effective immune elimination of SV40-transformed cells in vivo. More recently we have shown that in C3H mice the anti-SV40 CTL response is directed at only two small regions of the 90K SV40-encoded tumor (T) antigen. Two approaches are proposed to define further the CTL recognition sites on T antigen. First, deletion mutants that subdivide the target sites will be prepared by in vitro mutagenesis of cloned SV40 DNA and mouse cells expressing these mutants tested for lysis by a panel of Kk-restricted anti-SV40 CTL clones. Second, peptides corresponding to portions of the target sites will be synthesized. Antibodies to these peptides will be used in attempts to inhibit cytolysis by cloned anti-SV40 CTL and the peptides themselves will be used to stimulate CTL proliferation as final confirmation of the identity of the CTL recognition site. In addition, factors influencing determinant selection by anti-SV40 CTL will be evaluated. Specific factors to be considered include the influence of H-2 and non-H-2-linked genes, intramolecular antigenic competition whereby individual T antigen epitopes compete for association with the K/D restriction element, and physical constraints due to positioning of T antigen in the plasma membrane. Together these studies are designed to evaluate the rules by which CTL select immunogenic epitopes within a tumor-associated antigen with the long-term goal of developing strategies for the manipulation of the antitumor immune response.